The present invention relates to a process for manufacture of metallic magnesium, and, more particularly, relates to an improved process for manufacture of metallic magnesium, by thermal reduction of magnesium oxide, which can produce metallic magnesium of high quality at a relatively low cost, efficiently, and economically.
The following types of prior art processes are already known for manufacturing metallic magnesium: carbon reduction processes, carbide reduction processes, and silicon reduction processes.
Carbon reduction processes, such as the Hansgirg process and so forth, involve causing magnesium oxide and carbon to react at high temperatures according to the equation (1) below, and the resulting metallic magnesium vapor generated by this reaction is rapidly cooled, thus forming solid metallic magnesium. EQU MgO+C=Mg(gas)+CO (1)
The disadvantages of these carbon reduction processes are twofold. First, the metallic magnesium obtained from these processes is in the form of a fine dust, with particles of diameters from about 0.1 to about 0.6 microns; and according to this there is great difficulty in collecting and post processing the metallic magnesium which is produced. Second, the high temperature gaseous metallic magnesium which is produced is very liable to undergo the reverse reaction to equation (1), i.e. to be reconverted back into magnesium oxide, and therefore while collecting the metallic magnesium it is necessary to provide high speed cooling for said metallic magnesium. Two ways are known for providing this high speed cooling: the gas cooling process using hydrogen or natural gas, and the so called adiabatic self cooling process using a divergent nozzle. In the gas cooling process, the fact that a large volume of such gaseous coolant is required means that the manufacturing cost of the final metallic magnesium becomes very high. On the other hand, in the adiabatic self cooling process using a divergent nozzle, in order to overcome the fact that the metallic magnesium is in the above described fine dust form with particles of diameters from about 0.1 to about 0.6 microns, and in order to prevent the reverse reaction to equation (1) occurring, and in order to obtain a high grade of metallic magnesium as a final product, a high vacuum of from about 0.1 to about 0.3 torr is required. This necessitates the provision of a high capacity vacuum pump; and this is expensive, and also during production of metallic magnesium uses a lot of energy to operate, which is also costly. Because of these disadvantages, i.e. because of the basic cost of the process and because of the troublesome form of the metallic magnesium finally produced, the carbon reduction type of process for production of metallic magnesium has not been commercially used since 1945.
Carbide reduction processes, such as the Murex process and so forth, involve causing magnesium oxide to be thermally reduced using calcium carbide (CaC.sub.2) as a reducing agent. Now, it is possible to obtain metallic magnesium in this case, similarly as in the above described case of the carbon reduction process. However, the disadvantages of these carbide reduction processes are as follows. First, the cost of the calcium carbide required for reduction of the magnesium oxide is high. Second, a large amount of calcium oxide is produced after the reduction reaction as a troublesome sludge, which needs to be disposed of. Third, the metallic magnesium produced is of rather low grade. Because of these disadvantages, i.e. because of the basic cost of the process and because of the low grade of the metallic magnesium finally produced, the carbide reduction type of process for production of metallic magnesium has not been commercially used since 1945, either.
Silicon reduction processes, such as the Pidgeon process, the I.G. process, the Magnetherm process, and so forth, are the types of process that are currently used for commercial production of metallic magnesium. These processes, however, have the disadvantage that in order to produce one ton of metallic magnesium they consume from about 16,000 kWh to about 20,000 kWh of electrical energy, and produce about six tons of sludg. Accordingly, the final metallic magnesium produced is of rather high cost.